The present invention relates a color-separating and -recombining optical system having polarization beam splitters and a projection display using the optical system.
Color projection displays operate as follows: White light is separated into three primary colors R (Red), G (Green) and B (blue). The separated color components are guided to the corresponding spatial light modulators (abbreviated to SLM hereinafter) for optical modulation in accordance with a video signal. The modulated color components are recombined and projected onto a screen, thus displaying a color image thereon.
Color projection displays are classified into three types in accordance with SLMs to be used, such as, a type with SLMs, another with reflecting SLMs, and still another with a DMD (Digital Mirror Device).
Compact projection displays with transparent SLMs and DMDs having relatively simple optical architecture are available but have difficulty in resolution.
On the contrary, reflective SLMs exhibit high resolution but pose a problem in compactness due to complex optical system using this type of SLMs. Particularly, projection displays equipped with reflective SLMs require polarization beam splitters (abbreviated to PBS hereinafter) for splitting light beams incident to the SLMs and reflected light beams that have been modulated by the SLMs. In detail, each reflective SLM requires two or more of PBSs for high contrast, thus resulting in complex optical architecture for reflective projection displays.
Colorlink Inc. (US) has proposed a color-separating and -recombining optical system having no problem on optical architecture in use of reflective LSMs, introduced in literature “High Contrast Color Splitting Architecture Using Color Polarization Filters” by Michael G. Robinson et., SID 00 DIGEST, 92-95(2000).
FIG. 1 is a plan view illustrating an optical architecture for a projection display 300 using reflective SLMs, proposed by Colorlink Inc.
A color-separating and -recombining optical system 290 (enclosed by a dot line) has cubic- or square column-like first to fourth PBSs 102, 103, 104 and 105 arranged such that polarization-splitting planes 121, 131, 141 and 151 intersect each other almost like the character “X”.
First wavelength-selective polarizing converters (G-phase plates) 106 and 107 are provided on the light-incident plane side of the first PBS 102 (the left side of the PBS 102 in FIG. 1) and light-emitting plane side of the fourth PBS 105 (the right side of the PBS 105 in FIG. 1), respectively, for rotating the plane of polarization of a G-linearly-polarized light by 90 degrees.
Second wavelength-selective polarizing converters (R-phase plates) 108 and 109 are provided between the first and the third PBSs 102 and 104, and the third and the fourth PBSs 104 and 105, respectively, for rotating the plane of polarization of a R-linearly-polarized light by 90 degrees.
Linearly-polarized light is classified into S-polarized light and P-polarized light. A polarized light is decided as S- or P-polarized light in accordance with relativity between its plane of polarization and a polarization-splitting plane of a PBS to which it is incident. In other words, a polarized light is called S-polarized light when its plane of polarization is orthogonal to an incident plane against a polarization-splitting plane of a PBS, whereas it is called P-polarized light when its plane of polarization is horizontal to the incident plane.
The projection display 300 has a relatively simple optical architecture for high contrast even though it requires three PBSs for each reflective SLM.
Nonetheless, this projection display has a problem of low contrast at the corners of a black image screen due to birefringence caused by a transparent material for the PBSs due to wrong choice for the transparent material in the projection display 300 when a high-intensity discharge lamp of 100 W or more is used.
Japanese-Unexamined Patent Publication No. 9-54213 discloses that a transparent material of 1.5×10−8 cm2/N or less as the absolute value of opto-elastic constant is suitable for such PBSs.
It is disclosed that a transparent material of low opto-elastic constant is suitable at least for a main (reflective) PBS that splits incident light and light reflected therefrom after modulation.
The inventors of the present invention have, however, found that the problem discussed above cannot be solved by employing such transparent material of low opto-elastic constant when it is used only for the main PBSs (the second and the third PBSs 103 and 104) for the projection display 300 equipped with the color-separating and -recombining optical system 290.
The above problem could be solved by employing such transparent material of low opto-elastic constant when it is used for all of the four PBSs, which, however, results in high cost for the color-separating and -recombining optical system.
Such transparent material of low opto-elastic constant is generally several times or several ten times more expensive than usual optical glass such as BK7 because it contains much lead and hence too weak and soft for machining.
Moreover, the color-separating and -recombining optical system 290, offered by Colorlink Inc., has revealed low reliability because all optical elements of the optical system 290 joined by an adhesive were peeled off from each other at a thermal-cyclic reliability test.
The following is a possible reason for low reliability:
As already described, the color-separating and -recombining optical system 290 has four PBSs 102, 103, 104 and 105 arranged such that their polarization-splitting planes 121, 131, 141 and 151 intersect each other almost like the character “X”.
In the reliability test, the optical elements were subjected to thermal expansion and contraction while the optical system 290 were being heated and cooled cyclically. Stress was then generated from the center of the intersection of the four PBSs in the direction of circumference due to thermal expansion and contraction. The circumferential stress could cause outward shear stress in heating whereas tensile stress in cooling at each joint section of the optical elements, thus resulting is peeling-off for the optical elements from each other.
The character-“X”-like arrangements of the PBSs 102, 103, 104 and 105 also poses the following problem:
As illustrated in FIG. 2, some components of light incident to the first PBS (light-incident-side PBS) 102 are further incident to the fourth PBS (light-emitting-side PBS) 105. The unnecessary light components L are projected onto a screen (not shown) via a projection lens 191, to generate bright portions on the screen, thus resulting in low quality for images displayed thereon. The image quality will be further lowered when the four PBSs 102, 103, 104 and 105 are bonded each other by a joint material 110 such as a transparent adhesive.
When the color-separating and -recombining optical system 290 has integrators on reflective SLMs 161, 162 and 163 at the light-source side, an integrator-segment image is displayed on screen while light is illuminating these SLMs. However, light components spread over the periphery of each reflective SLM could also become the unnecessary light components L projected onto the screen.
In addition, light components reflected from the reflective SLMs 161, 162 and 163 could be reflected again at a first polarizing plate 181 and become the unnecessary light components L projected onto the screen.